An image forming apparatus includes a rotator (such as a photoconductor or a paper conveyer roller) provided to form an image on the rotator or on a recording medium moving with rotation of the rotator. In an electrophotographic printer, for example, an electrostatic latent image is formed on a rotating photoconductor by optical scanning, and thereafter is developed and transferred to a recording medium.
If the photoconductor rotates at a constant speed, line scanning at a constant time interval enables a proper image (as an electrostatic latent image, or a developed or transferred image), in which the scanning line interval is uniform.
However, the photoconductor actually has cyclic variation in rotational speed. This could result in an odd image, in which the scanning line interval has variation. Thus, image quality may be degraded due to the rotational variation of the photoconductor.
In view of this, it has been proposed that an image forming apparatus includes a function for suppressing variation in scanning line interval caused by variation in rotational speed of the photoconductor.
In the image forming apparatus, correction amounts corresponding to some phase points of rotation of the photoconductor are preliminarily measured, and the measurements are stored in a memory. The correction amounts are amounts of time used for correcting the scanning line interval at the respective phase points into a predetermined reference interval.
Specifically, the image forming apparatus starts line scanning of the rotating photoconductor, in response to an instruction for image formation. During the line scanning, the image forming apparatus regularly estimates the current phase of rotation of the photoconductor, based on detection of the origin phase of the photoconductor by an origin sensor, and further based on an internal clock provided therein.
The above correction amounts are sequentially retrieved according to the estimated current phase. Thereby, the starting time for each scanning line is corrected based on the retrieved correction amounts, so that the scanning line interval is consistently adjusted to the reference line interval.
However, the current phase, estimated based on the detected origin phase and the internal clock as described above, is not necessarily consistent with the actual current phase of the photoconductor. Further, the difference between the estimated current phase and the actual current phase may increase over the cycles of rotation of the photoconductor.
Consequently, the correction amount corresponding to a phase point substantially different from the actual current phase may be retrieved and used for correction, resulting in false correction. Thus, there is a problem that the effect of variation in rotational speed of the rotator cannot be adequately suppressed.
Note that the problem is still relevant to other kinds of printers than an electrophotographic printer, such as an ink-jet printer or a thermal printer (which thermally forms an image using a thermal paper and an ink ribbon), because the problem is due to variation in rotational speed of the rotator.
Thus, there is a need in the art to provide an image forming apparatus capable of suppressing the effect of variation in rotational speed of a rotator on image quality.